Demand for wireless communications equipment and services continues to grow at an unprecedented rate throughout the world. Increasingly, such systems are commonly relied upon to provide voice and data communications to a growing sector of the public. While these systems originally depended upon analog signaling technology, there is essentially unanimous agreement that future systems will be based on various types of digital signal coding schemes.
The typical wireless communication system is a point-to-multipoint type system in which a central base station communicates with a number of remote units located within a local geographic area of coverage known as a cell. This system provides for duplex communication such that signals may be sent in both a forward direction (from the base station to the remote unit) as well as in a reverse direction (from the mobile remote unit back to the base station). In order to support communication between the remote unit and networks such as the Public Switched Telephone Network (PSTN), or data networks such as the Internet, the wireless system must also provide for various logical components and functional entities.
Consider the Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA) digital systems presently in widespread use. Each of these systems provides for certain logical types of the radio channels that make up the forward link and reverse link. In particular, the forward link channels often include a pilot channel, paging channels, and multiple forward traffic channels. The traffic channels are used to carry the payload data between the base station and the mobile unit. A pilot channel is also typically required to allow the remote unit to maintain synchronization with the base station. The paging channels provide a mechanism for the base station to inform the remote unit of control information, such as the assignment of forward traffic channels to particular connections and/or subscriber units.
Likewise, an access channel is provided in the reverse direction in addition to reverse traffic channels. The access channels allow the remote units to communicate control information with the base station, such as to send messages indicating the need to allocate or deallocate connections as required.
Various. environmental conditions can affect the performance of any wireless communications system. These elements include atmospheric signal path loss, which may often introduce fading and interference. Fading may include variations that are introduced as a result of the specific terrain within the cell, as well as other types of fading, such as multipath fading, that occurs due to signal reflections from specific features, such as buildings that cause fluctuations in received signal strength. Systems in which the remote unit may be a mobile unit, especially those potentially operating at higher speeds, such as the cellular telephones used in automobiles, are particularly susceptible to multi-path fading. In such an environment, the signal pathways are continually changing at a rapid rate.
A similar impact on performance can result from movement of the subscriber units relative to the base station. Motion can make it difficult for the base station to precisely locate the subscriber unit. In addition, the signal pathways continuously change at a rate proportional to the mobility rate. To maintain a wireless link, additional power and traffic channels may have to be allocated to the moving subscriber. This additional allocation of wireless resources removes resources that would otherwise be available to other subscribers. This negative impact on other subscribers is more acutely felt during rapid movement and when attempting to maintain high data rates across the wireless link.